An Algebraic Characterization of Strictly Piecewise Languages
Jie Fu, Jeffrey Heinz, and Herbert G. Tanner
University of Delaware {jiefu,heinz,btanner}@udel.edu
Abstract. This paper provides an algebraic characterization of the Strictly Piecewise class of languages studied by Rogers et al. 2010. These language are a natural subclass of the Piecewise Testable languages (Si- mon 1975) and are relevant to natural language. The algebraic character- ization highlights a similarity between the Strictly Piecewise and Strictly Local languages, and also leads to a procedure which can decide whether a regular language L is Strictly Piecewise in polynomial time in the size of the syntactic monoid for L.
1 Introduction
Rogers et al. [12] study the Strictly Piecewise (SP), which are a proper subclass of the Piecewise Testable (PT) languages of Simon [13]. The Strictly Piecewise languages are interesting for two reasons. First, there are several senses in which the SP class is natural. For example, SP is exactly the class of those languages closed under subsequence [12]. Also, they bear the same relation to Piecewise Testable languages that the Strictly Local (SL) bear to Locally Testable (LT) languages [10, 12]. Second, this class expresses some of the kinds of long-distance dependencies found in natural language [6, 12]. While Rogers et al. provide several characterizations of SP languages, they do not provide an algebraic one. Also, the procedure they give for deciding whether a regular language L belongs to SP is exponential in the size of the smallest deterministic acceptor for L. This paper aims to address these issues. An algebraic characterization for the SP class is provided. This result not only reveals an important similarity between the SP and SL languages, but also leads to a procedure which decides whether L belongs to SP in time quadratic in the size of syntactic monoid for L. However, it remains an open question whether a polynomial time decision procedure exists in the size of the smallest deterministic acceptor. The rest of this paper is organized as follows. Section 2 reviews foundational concepts and notation. Section 3 defines the Piecewise Testable (PT), Strictly Piecewise (SP), and Stricly Local (SL) classes. Section 4 presents our algebraic characterization of the SP class and Section 5 describes the polynomial-time decision procedure. Finally, Section 6 concludes.